Fluid-operated control apparatus

ABSTRACT

The specification describes a rotary summating apparatus comprising a rotary disc which is arranged to be rotated by a fluid pressure-mechanical transducer in steps. Pressures whose values are to be added together are fed in sequence to the transducer so that the rotary part moves in steps proportional to the values of the pressures.

United States Patent [72] Inventor Gerhard Klee Frankfurt amMain-Ginnheim, Germany [21] Appl. No. 867,780 [22] Filed Oct. 20, 1969[45 Patented Jan. 11, 1972 [73] Assignee Samson Apparatebau A.G.

Frankfurt am Main, Germany [32] Priority Sept. 5, 1966 [3 3 Germany [3I] S 105696 Original application Sept. 5, 1967, Ser. No. 665612, nowPatent No. 3,489,064, dated Jan. 13, 1970. Divided and this applicationOct. 20, 1969, Ser. No. 867,780

[54] FLUID-OPERATED CONTROL APPARATUS 4 Claims, 16 Drawing Figs.

[52] US. Cl 137/83, 91/44, 91/388 [51] Int. Cl F151) 5/00 [50] Field ofSearch 137/83; 91/41, 44; 235/91 [56] References Cited UNITED STATESPATENTS 3,064,464 11/1962 Black 91/44 X 3,068,650 12/1962 Phillips 91/44X Primary Examiner-Alan Cohan Attorney-Watson, Cole, Grindle & WatsonABSTRACT: The specification describes a rotary summating apparatuscomprising a rotary disc which is arranged to be rotated by a fluidpressure-mechanical transducer in steps. Pressures whose values are tobe added together are fed in sequence to the transducer so that therotary part moves in steps proportional to the values of the pressures.

PATENTED JAN! 1 B72 SHEET 1 BF 8 Fig. 2

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I.\'VE.\ I ()RI G rhard K268 FLUID-OPERATED CONTROL APPARATUS Thisapplication is a division of application Ser. No. 665,612, filed Sept.5, 1967, now US Pat. No. 3,489,064, issued Jan. 13,1970.

The present invention relates to control systems and more particularlyto pneumatically operated regulators and servomotors in which the inputand output quantities are compared with each other to find the deviationor error between the two quantities.

In previously proposed regulating equipment exhibiting proportionalintegral behavior comparison between input and output quantities isgenerally carried out simultaneously, that is to say an element issimultaneously responsive to both quantities and its instantaneousposition or state represents the instantaneous difference or sum of thetwo quantities. In many cases such a construction leads to complexity.If, for example, the output quantity is discontinuously measured, asoccurs in manufacturing and process technology, known regulators must beconnected with additional devices or equipment, for example for storingthe output quantity.

The present invention consists in a control system comprising means foracting on a controlled member, means for sensing an output quantity ofthe controlled member and providing a corresponding output quantitysignal, a comparison element arranged to move in response both to theoutput quantity and an input quantity signal to provide a signal forcontrolling action of the controlled member, means for repeatedlylocking such movement of the comparison element and means for connectingthe comparison element alternately with the input and output signalsrespectively during periods when the comparison element is free to move,the comparison element being arranged to move in accordance with adifference in magnitude of the input and output quantity signals and toremain locked after so moving.

Preferably the system includes means for acting on a controlled member,means for sensing an output quantity of the controlled member andproviding a corresponding output quantity signal, a comparison elementarranged to move in response both to the output quantity and an inputquantity signal to provide a signal for controlling action of thecontrolled member, means for repeatedly locking such movement of thecomparison element and means for connecting the comparison elementalternately with the input and output signals respectively duringperiods when the comparison element is free to move, the comparisonelement being arranged to move in accordance with a difference inmagnitude of the input and output quantity signals and to remain lockedafter so moving.

The system can further comprise a mechanical-fluid pressure transducerarranged to be operated by one of the two movable parts. Themechanical-fluid pressure transducer can comprise a vane arranged to actupon a jet of fluid while the fluid pressure-mechanical transducerconnected with the two movable parts is in the form of a bellowselement.

In accordance with a still further preferred feature of the inventionthe two movable parts are in the form of aligned rods extending inopposite directions from the fluid pressuremechanical transducer and theclamping means are arranged to bring pressure to bear on the rods in adirection substantially towards the axes to the rods, the clamping meansbeing fluid pressure operated.

One such rod can be arranged to operate a mechanical-fluid pressuretransducer and the system can further comprise fluidpressure-operatedcoarse setting means arranged for moving the other rod. The bellows canbe arranged to be acted on firstly on both sides by one of the quantitysignals and then by the two quantity signals on both of its sidessimultaneously.

In accordance with a further form of the invention the two movable partsare in the form ofa hollow rod and a rod fitting inside it, the two rodsextending in the same direction from the fluid pressure-mechanicaltransducer, the system further comprising two sets of clamping jawsarranged to engage the two rods respectively, screw-threaded means forfixing the jaws in the sets of jaws, and pressure-operated means foroperating the screwthreaded means. The system can further comprise acutting tool connected with one of the rods.

In accordance with a still further embodiment of the invention themovable parts are arranged to rotate and the fluid pressure-mechanicaltransducer is arranged to produce relative rotational movement betweenthem.

In one form of the invention in which the comparison element comprisesmovable parts which are arranged to rotate, the parts are in the form ofcircular discs and the system further comprises radially actingpressure-operated brake means for engaging the peripheries of the discs,and a vane arranged to be rotated by one of the discs for controlling anozzle.

In accordance with a still further construction in accordance with theinvention the comparison element is in the form of a single-rotary partcarrying a scale, the system further including brakes for acting on theperiphery of the driven part and brake and clutch means for connectingan arm, which is arranged to pivot co axially with the driven part, withthe driven part and releasing it from it.

In systems in accordance with the invention the comparison element ismoved in steps in one direction or the opposite direction in accordancewith each controlled deviation between the input and the outputquantities sensed, the direction of the displacement depending upontheprefix sign of the controlled deviation while the size of thedisplacement depends upon the amount of the controlled deviation takinginto account the P-range of operation of the controller.

A system in accordance with the invention can not only be used forcarrying out control operations with discontinuously measured controlquantities, but also for control operations in the case of continuouslymeasured control quantities, such as temperature or pressure values,measuring means in the latter case being fed with the aid of anysuitable control means alternately with the input and output quantitiespresent at the input or inlet of the regulator. In both cases theinvention enables an additional regulator to be dispensed with.

Various embodiments of the invention are now described with reference tothe accompanying drawings.

FIGS. 1 and 2 diagrammatically show a regulating circuit with aregulating device showing application to a machine tool.

FIG. 3 is a diagrammatic longitudinal section through a regulatingdevice in accordance with a first embodiment of the invention;

FIG. 4 is a cross section ofa clamp for the device of FIG. 3; FIG. 5 isa table of the regulating steps for the device of FIG.

FIG. 6 shows a diagrammatic longitudinal section through a regulatingdevice in accordance with a second embodiment of the invention;

FIG. 7 is a table ofthe regulating functions of FIG. 6;

FIG. 8 is a partial longitudinal section of a regulating device inaccordance with a third embodiment of the invention;

FIG. 9 is an enlarged cross section ofa part of the device of FIG. 8;

FIG. 10 shows a partial longitudinal section of a regulating device fora lathe or similar machine tool in accordance with a fourth embodimentof the invention;

FIG. 11 shows in perspective and partiallyin section a re gulatingdevice in accordance with the invention in which the measuring meansmoves angularly;

FIG. 12 is a table of the operating steps of the regulating equipment ofFIG. 11;

FIGS. 13 and 14 show a further embodiment of a device in accordancehaving the invention with a rotary servomotor;

FIGS. 15 and 16 are tables representing the operational steps of theapparatus in FIGS. 13 and 14.

The regulating circuits schematically shown in FIGS. 1 and 2 representtwo embodiments of the invention with discontinuous measurement of thecontrol quantities with reference to a control process in which thediameter of workpieces being turned on a lathe is to be regulated. Inthis arrangement the output quantity of a finished workpiece l isdetermined by a pneumatic measuring head 2 and fed to an incrementalregulator 3 via a duct 5. This incremental regulator compares theinstantaneous output quantity with the input quantity which is indicatedeither pneumatically via a duct 4 or mechanically by means of anadjustable spring or the like. Within the incremental regulator 3 thecontrol deviation determined is converted by means of an outputtransducer into a pneumatic output quantity which is fed via a duct 6 toa servomotor 7 as a servocontrol signal. The pneumatically operatedservomotor 7, which can be constructed in a known manner, is arranged onthe lathe slide 8 of an automatic lathe and acts on the tool carrier 12whose tool 9 is machining the workpiece 10. When the servo-operatingsignal or quantity changes owing to a control deviation, the stroke ofthe servomotor 7 is altered and with it the setting of the turning tool9. Consequently the workpiece 10 held in the lathe is turned to anaccordingly greater or smaller diameter, the control or regulating cyclebeing repeated as required until the workpiece corresponds with theinput quantity.

The regulating device shown in FIG. 2 differs from that of FIG. 1substantially in that the incremental regulator 11 also serves as aservomotor. The output signal of the regulator 1] acts directly, that isto say without a pneumatic output device, on the tool holder 12 and thuson the turning tool 9. In other respects the manner of operation of thisregulating circuit corresponds with that of the regulating circuitaccording to FIG. 1.

In the case of the regulator shown in FIGS. 3 and 4 a measuring means 15with a metal bellows is used whose parts 16 are held together by meansof strip springs 17 and whose end walls are connected with rods 18 and19. Rods l8 and 19 can be held in clamps 20 and 21 and constitute thecomparison element of the system. Each clamp consists of a block 25fixed on the baseplate 26 of the regulator and has a groove in which therods 18 and 19 are held. The rod can be clamped by means of a spring 27between a jaw 28 and the groove of the block 25. The jaw 28 is connectedby means of a piston rod 29 with a piston 30 sliding in a cylinder 32.When the cylinder space 31 below the piston 30 is put under pressure,the clamp is released and the rods 18 and 19 can be moved axially. Thecylinder is connected firmly with the block 25 by means of massivestraps. At the free end of the rod 19 for the output signal of themeasuring means 15 a vane 36 is attached sliding between the blowing andreceiving nozzle 37 and output device 35. This output device constitutesa mechanicalfluid pressure transducer. The blowing and receiving nozzlefed by the supply duct Z are arranged in the cover of a spring mountedmetal bellows 38 whose end 39 is mounted on the base plate 26 of theregulator. The output pressure is supplied from the metal bellows 38 indirection of the arrow A, for example, through the duct 6 of the controlcircuit shown in FIG. 1 to the servomotor 7. The pressure measuringmeans 15 is supplied with the input quantity W and the output quantity Xsequentially, that is to say one after the other, by means ofa switch S.In the input part of the pressure measuring means 15 there is also apressure-distributing circuit which consists of a constant value choke42 and an adjustable choke 41 and serves for setting the proportionalrange of the controller. Instead of this, the proportional range of theoutput device 35 can be made adjustable. The cycle for comparing theinput and output quantities with this device is carried out in a numberof sequential steps shown in the table of FIG. 5.

During the step I the clamp 20 is released, that is to say the rod 18 ofthe measuring means 15 can slide freely, and the clamp 21 is tightenedso that the rod 19 of the measuring device 15 is held firmly. In thetable of steps a minus sign is used to indicate released and a plus signto indicate tightened. If switch S is in the position W, so that themeasuring means 15 receives the input quantity, the measuring meansconsequently makes a movement to the left corresponding to the inputquantity. During the two following steps II and III both clamps 20 and21 are tightened so that the measuring means 15 cannot move. However, onpassing from step II to step II the switch S is switched over from theposition W into the position X. In this position, shown in FIG. 3, themeasuring means 15 receives the output quantity X. In step IV the clamp21 is released, that is to say the rod 19 is freed. If the input andoutput quantities are now equal, the measuring device is not moved andthe position of the rod 19 is also not changed. ll, however, the outputquantity is greater than the input quantity, corresponding to a positivecontrol deviation, the measuring means 15 moves the rod 19 to the right.If on the other hand the output quantity is smaller than the inputquantity, corresponding to a negative control deviation, the measuringmeans 15 draws the rod 19 back towards the left. The position reached bythe rod 19, forming the output signal to the measuring device 15, mustnow be fixed. For this purpose in step v both clamps 20 and 21 aretightened. The switch S, which in step V is at first connected with theoutput quantity X, is now switched over in step VI to be connected withthe input quantity W. With this the comparison cycle is completed. Theterminal position of the measuring means, the position of the rod 19,remains fixed without alteration until the next comparison between theinput-output quantities begins again in step 1. During the change inposition of the rod 19 the position of the vane 36 in the nozzle system37 of the output unit 35 varies so that the displacement is transformedinto a change in pressure proportional to the movement of the rod 19. Inthis manner a corresponding pneumatic output signal of the regulator isobtained. The control vane 36 is arranged in relation to the output unit35 in the input-output quantity comparison cycle in such a manner that apositive control deviation corresponds with a reduction in the outputpressure at A, that is to say, the direction of operation of theregulator in this case is rising/falling. A reversal in the direction ofoperation so as to be rising/rising, in which a positive controldeviation corresponds with an increase in the output pressure, isbrought about in a simple manner by changing the association between theactuation of the clamps 20 and 21 and the position of the switch S. Inthe two last vertical columns of the table of FIG. 5 the switchpositions for these two directions of operation are indicated. Thecontrol of the regulator shown in FIGS. 3 and 4 in accordance with thistable can be brought about with a known following control which is notshown in the drawing.

The embodiment of the regulating device illustrated in FIG. 6, insteadof the pressure measuring means 15 of- FIG. 3, is provided with adifferential pressure measuring means 45 in which the control deviationsare determined. A membrane 48 is mounted in a control housing 47 betweenthe springs 49. As was the case with the device of FIG. 3, a rod 18 isconnected with the housing 47 and is mounted in a clamp 20 while the rod19 passing through the housing 47 and connected with the membrane 48 ismounted in the clamp 21. At the free end of the rod 19 the control vane36 of an output unit 35 is mounted as is shown in FIG. 3. The rod 18 isextended to the left beyond the clamp 20 and connected with anadditional driving system 46 which enables a rapid movement of themeasuring means as far as the terminal position, or a similar travel ofthe measuring means to be carried out. This drive system consists of arolling membrane or cuff 52 mounted between a housing 50 and a housingcover 51, a membrane carrier 53, and a spring 54. The housing 50 isconnected with the baseplate 26 of the regulator. The free end of therod 18 inserted into the housing 50 is connected with the membranecarrier 53. When the membrane or cuff 52 is acted upon by pressureentering through inlet 55 after both the clamps 20 and 21 have beenreleased, the rod 18 serves to displace the measuring means 45 and thusalso rod 19. For example the drive system 46 can be so constructed thatthe measuring means can be displaced with a variation in pressure of0.2-1.0 (atmospheric gauge pressure) from one end or terminal positionto the other terminal position. The manner of operation of thisregulating device can readily be understood with reference to the tableof steps and switching operations given in FIG. 7 and the explanationgiven in connection with the embodiment of FIG. 3. The positions of theswitch S which in one position transmits the input quantity W to bothchambers or spaces and in the other position transmits the outputquantity X into the one measuring means chamber while the othermeasuring means chamber remains connected with the input quantity, aregiven in the table again for both directions of the regulator.

The embodiment of the invention shown in FIGS. 8 and 9 is in the form ofa regulating device in which a deviation from a desired control functionis formed in a measuring means 60 and converted into a displacement andtransformed by means of an output unit 61 into a pressure proportionalto the control deviation. The two clamps 62 and 63 mounted on theregulator base plate 64 can be constructed as was the case with theprevious embodiments of the invention or in accordance with FIG. 9. Areceiving body 65, welded on the baseplate 64, has a receiving groove 66for the rod 67 or 68 to be clamped. On the receiving body 65 a holdingarm 70 is held by means of a spring band 69. Through a hole in thisholding arm 70 there extends the upper end of a bolt which is fixed inthe receiving body 65 and has on its thread a nut which can be screwedto act upon Belville washers 73. The nut 72 can be used to tighten therod 67 or 68. This can be carried out pneumatically as shown in FIG. 8.The nuts 72 of the two clamps 62 and 63 are for this purpose connectedby levers 75 and coupling rods 76 with a double-membrane system 77. Whenthe membranes 78 and 79 are acted upon by the pressure introduced intothe chambers 80 and 81, the nuts 72 are pulled tight and the clamps 62and 63 tightened. Conversely, the clamps are released if the membranesare acted upon by pressure introduced into the chambers 82 and 83. Sinceall chambers can also be fed with pressure independently of one anotherit is also possible to tighten or release only one of the clamps 62 and63. The rod 67 is connected with the membrane plate 86 of the measuringmeans 60 and is mounted in the clamp 63 so as to be movable. It alsocarries a control vane 87 for the output unit 61. The rod 68 has a holein the longitudinal direction and is firmly connected with the membranehousing 88. The rod is mounted in the clamp 62, the rod 67 passingthrough the hole in the rod 68. The method of operation of this controldevice basically corresponds with that of the regulator in accordancewith FIG. 3 with the only difference that, instead of the metal bellowstype measuring means 15, use is made of a measuring means 60 with aspring loaded membrane 85 acted upon by the pressure coming through theinlet 89 in correspondence with the input or output quantity.

In the construction in accordance with FIG. the regulator also forms theservomotor. The membrane system or measuring means 95 in this casepreferably includes steel membranes 97 which owing to machined awayparts in the zone 98 are so dimensioned that they also form membranes.These membranes are very stiff so as to ensure that the counter forcesdue to the working tool 99 on the tool carrier 96 can only affect themeasuring means 95 to a sight degree. The membranes 97 therefore onlymove through small distances. Preferably several such membranes 97 arearranged one behind the other in order to increase the thrust produced.The output signal from the measuring means 95 in this case determinesmechanically in a direct manner the setting of the workpiece carrier 96attached to the rod 67 and thus the position of the working tool 99 inrelation to the workpiece 100.

Embodiments of the invention in which the measuring means performsrotational or angular movements instead of translatory ones offer theadvantage that the moving parts of the device can be balanced as toweight and can be directly supported so that bearing errors anddeviations, for example due to vibrations, can be avoided. A furtheradvantage of the use of a system with angular movements is that themovement of the servomotor is not limited. One can thus arrange thesystem so that the angular movement is greater than 360.

A device with rotary or angular movement of the measuring means can forexample be constructed in accordance with the invention in such a mannerthat the measuring means con nects two rotary terminal parts, such asdiscs or similar bodies, in such a manner that variations in pressure inthe measuring means bring about alterations in the angle of one end partin relation to the other. The corresponding rotary end parts, such asdiscs or the like, are held by clamps or the like or released by them,in accordance with a further important feature of the invention. Theangular movement of one rotary end part, for example one of the discs,is converted by means of an output unit into a proportional pressure ina manner similar to a device working with translatory movement.

In accordance with a further advantageous embodiment of the inventionwith rotary or angular movements, the device is provided with a rotaryterminal part, for example a discshaped or similar body, which can beheld or released by a clamp, the pressure measuring means being capableof being connected by means of a clamp with the end or terminal part,for example the disc, while the clamp iscapable of being released bymeans of a pressure-responsive part.

Apparatus in accordance with the present specification can also, inprinciple, be used for carrying out calculation operations such as forexample addition and subtraction. With a device in accordance with theinvention it is thus possible to carry out summational operations of alltypes or determine averages. This means that the device can for instancebe applied to the measurement of quantities of liquids in a number ofcontainers by adding the levels. A particularly advantageous field ofapplication for their respective invention is in the counting ormeasurement of quantities of flowing media. In a construction designedfor this purpose in accordance with the invention the operational stepsmust be precisely timed and the input pressure must be proportional tothe indication ofa flow meter or the like. For this application a devicewhose measuring means forms rotary movements is particularlyadvantageous because a counter can be driven directly. Also the totalcounting of a number of flow stations can be carried out comparativelysimply in accordance with the invention. In this case the step cycles ofthe individual flow meters, whose values are to be compiled, areconveniently arranged so as to occur in sequence and for determining thenumerical value the total tim of all individual cycles is sued.

Some examples of devices in accordance with the invention in which themeasuring means forms angular or rotary displacements and thedisplacement steps are angular rotations instead of translatorymovements, are shown in FIGS. 11, 13, and 14 with the respectiveassociated table of steps shown in FIGS. 12, 15 and 16.

FIG. 11 shows a device corresponding in principle to the deviceaccording to FIG. 3 but arranged to perform angular movements instead oftranslatory ones. The rotary discs or wheels 118 and 119 are connectedtogether by the pressure measuring means in such a manner that a changein pressure in the measuring means 115 causes a change of the angle ofthe disc or wheel 119 in relation to the disc 118. The latter disc canbe held by a clamp 120 while the disc 119 can be held by a clamp 121because in this case a spring 127 presses the brake shoe 128 against theperiphery of the corresponding wheel or disc. However, if the pressureis transmitted to the cylinder chamber 131, the spring 127 is compressedso that the brake shoe 128 is raised and the corresponding disc isreleased for angular movement. The angular movements of the disc 119 areconverted by means ofan output unit into a proportional pressure at A.For this purpose, for example, a spiral-shaped disc 136, which isconnected so as to rotate with the disc 119, is mounted in the slot ofablowing and receiving nozzle system 137 arranged on the head of aresilient bellows 138. The bottom 139 of the resilient bellows remainsstationary. I

The manner of operation of this device can easily be understood from thetable of FIG. 12 and the description of FIG. 3. FIGS. 11 and 12 show thearrangement for an increasing pressure at A when the pressure of theoutput quantity is greater than the input quantity. A reversal can beachieved either by reversing the switch positions W and X of the switchS (input and output 136.

A further device with a rotating terminal servo part is diagrammaticallyshown in FIGS. 13 and 14. FIG. 13 shows the quantities) or by reversingthe spiral disc device from the side, the drive of the clamps 170 beingomitted for the sake of clarity. FIG. 14 shows a view in which theclamps 160 are omitted for the same reason. FIG. 15 shows thecorresponding table of operational steps.

A disc or wheel 151 is connected with a shaft 152 and the two can rotatewhen they are released by the clamp 160. The clamp consists of twolevers 153 each with a brake shoe 154 pressed by a spring 155 againstthe periphery of the ,disc 151 so as to prevent the disc from rotating.The levers 153 are also connected by a cylinder 156 and piston 157. Ifpressure is communicated to the cylinder space or chamber 158, thebraking levers 153 are pushed away from each other against the force ofthe spring 155 and the disc or wheel 151 is released. The pressuremeasuring means 180, which consists of a spring loaded membrane, isconnected to lever 181, pivoting on a shaft, by means of a link 182. Thelever 181 carries a clamp 170 by means of which it can be clamped by theaction of a spring 173 on the disc 151. The clamp 170 is, however, alsoconnected with a piston 174 and a cylinder 175. When the cylinder space176 is put under pressure, the lever 177 is raised against the action ofthe spring 173 away from the clamp and the disc 151 is released from itsconnection with the lever 181. The operation step table of F I6. 15explains the manner of operation of this device when the latter is usedas a regulator.

The device can, however, also be used for other purposes such ascalculating, for instance addition and subtraction. In this case themeasuring means 180 is then fed with a pressure corresponding to thezero value instead of the input quantity and, instead of the outputquantity, a pressure corresponding to the value to be measured. Thepressures which are greater than the pressure corresponding to the zerovalue are then added and those which are smaller are subtracted. Thesetting of the disc or wheel 151 gives the result. The disc can beprovided with a scale or can drive a counting device directly. Insteadof the pressure measuring means 180 it is also possible to use adifferential pressure measuring means. On one side of the differentialpressure measuring means those pressures are fed which are to be addedwhile on the other side pressures are fed which are to be subtracted.

A further practical field of application of the invention is, forexample, the counting of quantities on the basis of flow measurements.The flow is measured for example in kg. per minute. If the flow ismeasured for each minute and the values are added together the quantityin kilograms flowing through the measuring device is obtained. Thedevice shown in FlGS. 13 and 14 is suitable for determining quantitiesif the stepping cycle is precisely timed. The input quantity is thenzero pressure and the output quantity is the pressure which correspondsto the instantaneous flow value, both these pressure values being fed tothe pressure measuring means. Preferably a counting device is provideddriven by the shaft 152.

A counting of several partial flows, which are measured by several flowmeters, can be carried out in accordance with a further embodiment ofthe invention. The table of operational steps of FIG. 16 shows themanner of operation. In the table, 0 denotes zero pressure while thenumbers indicate the flow measured at the various measuring positions.The duration of the whole cycle 18 steps) is used as a basis fordetermining the quantities counted. If the device is used fordetermining an average or mean or for the determination of the contentsof several containers by determining the differences in height, thedevice must be set at zero after each cycle, this being simple to carryout with known means.

Whatl claim is: l. A system for generating a pressure output in responseto first and second pressure inputs, comprising;

means for alternately sensing said first and second pressure inputs andgenerating respective corresponding output responses, means selectivelyrotatable in accordance with said output responses for comparing saidfirst and second pressure inputs with res ect to one another, means foreye ically locking said means for comparing to prevent rotation thereof,means for connecting said means for alternately sensing with said firstand second pressure inputs only during periods when said means forcomparing is free to rotate, driver means rotated in accordance with theselectable rotation of said means for comparison, and means forproviding said pressure output and operated by said driver means wherebysaid pressure output is proportional to said first and second pressureinputs.

2. A system as in claim 1 wherein said means for comparing includes twodiscs each independently rotatable through a predetermined angle inaccordance with a respective one of said output responses andinterconnected with each other by said means for sensing, whereby thecomparison of said first and second pressure inputs is represented bythe rotation of one of said discs relative to the other of said discs.

3. A system as in claim 1 wherein said means for cyclically locking saidmeans for comparing includes a pair of independently operable brakemeans, each one of said pair of brake means engaging an associated oneof said discs subsequent to a rotative movement thereofl 4. A system asin claim 3 wherein said driver means includes a cam mounted to rotatewith said one disc the rotation of which represents said comparison andsaid means for providing said pressure output is a resilient bellowsdevice having a bellows member engaging said cam to be moved thereby.

1. A system for generating a pressure output in response to first andsecond pressure inputs, comprising; means for alternately sensing saidfirst and second pressure inputs and generating respective correspondingoutput responses, means selectively rotatable in accordance with saidoutput responses for comparing said first and second pressure inputswith respect to one another, means for cyclically locking said means forcomparing to prevent rotation thereof, means for connecting said meansfor alternately sensing with said first and second pressure inputs onlyduring periods when said means for comparing is free to rotate, drivermeans rotated in accordance with the selectable rotation of said meansfor comparison, and means for providing said pressure output andoperated by said driver means whereby said pressure output isproportional to said first and second pressure inputs.
 2. A system as inclaim 1 wherein said means for comparing includes two discs eachindependently rotatable through a predetermined angle in accordance witha respective one of said output responses and interconnected with eachother by said means for sensing, whereby the comparison of said firstand second pressure inputs is represented by the rotation of one of saiddiscs relative to the other of said discs.
 3. A system as in claim 1wherein said means for cyclically locking said means for comparinginCludes a pair of independently operable brake means, each one of saidpair of brake means engaging an associated one of said discs subsequentto a rotative movement thereof.
 4. A system as in claim 3 wherein saiddriver means includes a cam mounted to rotate with said one disc therotation of which represents said comparison and said means forproviding said pressure output is a resilient bellows device having abellows member engaging said cam to be moved thereby.